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13.6.2 X-ray diffraction

X-rays for chemical analysis are commonly obtained byrotating anode generators

(in-house) orsynchrotronfacilities (Fig. 13.14). In rotating anode generators, a

rotating metal target is bombarded with high-energy (10–100 keV) electrons that

knock out core electrons. An electron in an outer shell fills the hole in the inner shell

and emits the energy difference between the two states as an X-ray photon. Common

targets are copper, molybdenum and chromium, which have strong distinct X-ray

emission at 1.54 A ̊, 0.71 A ̊ and 2.29 A ̊, respectively, that is superimposed on a

continuous spectrum known asBremsstrahlung. In synchrotrons, electrons are accel-

erated in a ring, thus producing a continuous spectrum of X-rays. Monochromators

are required to select a single wavelength.

As X-rays are diffracted by electrons, the analysis of X-ray diffraction data sets

produces anelectron densitymap of the crystal. Since hydrogen atoms have very

little electron density, they are not usually determined experimentally by this

technique.

Unfortunately, the detection of light beams is restricted to recording the intensity

of the beam only. Other properties, such as polarisation, can only be determined with

rather complex measurements. The phase of the light waves is even systematically

lost in the measurement. This phenomenon has thus been termed thephase problem

Storage
ring

Booster

ring

Electron gun

Beamlines

30m

X-ray

source

”Optics”

Monochromator

Focusing mirror

Collimator

Crystal

mounted on

goniometer

Beam

stop

Detector

Wavelength (Å)

X-ray spectrum generated
(a) (b) by a copper anode

(c)

10m

X-rays

0 0.5 1.0 1.5 2.0 2.5

Intensity

e–

Kβ

Kα

Fig. 13.14Instrumentation for X-ray diffraction. The most common X-ray sources are (a) particle storage

rings which produce synchrotron radiation, and (b) rotating anode tubes. The schematics of an X-ray

diffractometer are shown in (c).

547 13.6 X-ray diffraction